Insulation Resistance zero Ohms

[KeenPensioner]

Hi all. As I've said before, I'm not an electrician but an adult learner. I have a second hand Fluke 1653B MFT which is out of calibration but I use a CalCard to see how far out it is and it's not too bad. I've watched a thousand videos on EICRs etc and just want to increase my knowledge - if you want to kick me off this discussion as not being "competent" then fair enough. I will not be offering any electrical services to anyone - this is a hobby of mine.

Question: I carried out an IR test (at 250V) on one of my own ring (final!!) circuits protected by a 32A type B 30mA RCBO. L-E, N-E >200 mega ohms but L-N was zero ohms. There is nothing connected to any of the sockets. I've searched here and elsewhere for some info on this and obviously the possibility of a fault was mentioned. If there is a L-N fault why doesn't the RCD trip under short circuit fault?

 

[SJD]

L to N is not "zero" ohms, but some low value that shows up as zero because it is at the bottom of the scale. Change to a Continuity measurement, and likely you will see a reading of what is actually connected - e.g. an appliance not disconnected, etc. - e.g. a few tens or hundreds of ohms.

 

[Julie.]

Hi all. As I've said before, I'm not an electrician but an adult learner. I have a second hand Fluke 1653B MFT which is out of calibration but I use a CalCard to see how far out it is and it's not too bad. I've watched a thousand videos on EICRs etc and just want to increase my knowledge - if you want to kick me off this discussion as not being "competent" then fair enough. I will not be offering any electrical services to anyone - this is a hobby of mine.

Question: I carried out an IR test (at 250V) on one of my own ring (final!!) circuits protected by a 32A type B 30mA RCBO. L-E, N-E >200 mega ohms but L-N was zero ohms. There is nothing connected to any of the sockets. I've searched here and elsewhere for some info on this and obviously the possibility of a fault was mentioned. If there is a L-N fault why doesn't the RCD trip under short circuit fault?

It is more likely that there is something in circuit - perhaps a charger, or if one or more of the socket outlets have USB supplies, could just be a neon of course.

It depends on the tester, but it really isn't measuring 0 ohm, just lower than the minimum resistance on that range.

 

[littlespark]

Did you disconnect the tails from the rcbo before testing, or test at the terminal screws?

 

[James]

An rcd compares the current in the live and N conductors,

It will not trip on overload, even with a direct short from live to N the current in each cable is identical and the rcd will "see" no fault

a fuse or circuit breaker AKA mcb or mccb will trip on overload, it is what it is designed to do.

an rcbo, is a combination device that incorporates the functions of both an rcd and and mcb, it will trip on overload and/or trip when there is an imbalance between the live and N currents.

going back to your dilemma, the most likely "fault" that you have is something still plugged in or connected to the ring,
look out for rcd sockets, fused spurs for doorbells, cooker hoods, gas ignitors, outside lights, sheds, boilers, alarms, etc. etc.

 

[KeenPensioner]

Did you disconnect the tails from the rcbo before testing, or test at the terminal screws?

I tested at the RCBO terminal screws

 

[Marvo]

During an IR test the tester is using a high voltage to look for resistance readings in the millions of ohms range so, as mentioned above, if you're testing between L+N and anything is plugged into the circuit of if there's any neon power indicator lamps on any of the accessories the test result could show as zero Megaohms. This doesn't indicate a 'short' fault, it just indicates that the resistance is less than can be accurately measured by a tester that's looking for a result in the millions of ohms range.

Also as Little spark implied, if the circuit was still connected to the RCBO then you were testing in two directions. Not only were you testing the circuit wiring you were also testing backwards into the RCBO at the same time. The RCBO has internal circuitry that could skew the test and render the results suspect at best and completely meaningless at worst so always disconnect the circuit you're testing for this reason. Also if you accidentally use the 1000v IR testing range (some testers can even use 5000 volts) you could possibly internally damage the RCBO.

The RCBO is a device that also offers thermal and magnetic type overload protection as well as residual current so if you were to do a resistance test using the ohms range and the result was zero or nearly zero then yes, the RCBO would trip on overload if it was reset to this circuit condition.

 

[KeenPensioner]

During an IR test the tester is using a high voltage to look for resistance readings in the millions of ohms range so, as mentioned above, if you're testing between L+N and anything is plugged into the circuit of if there's any neon power indicator lamps on any of the accessories the test result could show as zero Megaohms. This doesn't indicate a 'short' fault, it just indicates that the resistance is less than can be accurately measured by a tester that's looking for a result in the millions of ohms range.

Also as Little spark implied, if the circuit was still connected to the RCBO then you were testing in two directions. Not only were you testing the circuit wiring you were also testing backwards into the RCBO at the same time. The RCBO has internal circuitry that could skew the test and render the results suspect at best and completely meaningless at worst so always disconnect the circuit you're testing for this reason. Also if you accidentally use the 1000v IR testing range (some testers can even use 5000 volts) you could possibly internally damage the RCBO.

The RCBO is a device that also offers thermal and magnetic type overload protection as well as residual current so if you were to do a resistance test using the ohms range and the result was zero or nearly zero then yes, the RCBO would trip on overload if it was reset to this circuit condition.

Thanks for taking the time to help. I understand the dangers of damaging electrical components and that's why I used a voltage of 250V (as I said - this is just practice for me). I turned the RCBO off during the test and I assumed that I was not sending a voltage through the RCBO, I am incorrect in that assumption? One of the videos I watched was from GSH Electrical and the sparky there did not disconnect the conductors from the RCD (at about 13 minutes in)

 

[Highresdis]

It may be possible to test at the terminals of some RCBOs, but it's not something I would do. In the case of a circuit connected to an RCBO, I will always remove both the live and neutral conductors before the insulation resistance test.

Try it. Let us know if there's a different result.

 

[pc1966]

Did you switch off / trip the RCBO before testing the output? Otherwise you will be testing L-N for the whole installation as even with the main switch off, the L & N busbars are comoning all circuits supplies.

Most insulation testers have a resolution of 0.01M = 10k ohm = 5-6W load, and at DC things like transformers or AC motors are far lower imedance at DC (few hundred ohms or less) than they appear when powered from AC.

 

[KeenPensioner]

Thank you all for taking the time to reply.
I have a lot of theory but little practical knowledge. I'll disconnect the conductors from the top of the RCBO (as everyone suggests) and see what I get.

 

[telectrix]

if you have something like a neon in circuit, your IR reading will be something like 0.02 Meg.Ohm, which, on that range is virtually Zero. however, a continuity reading would be more like 20K Ohms, which is a significant value.

 

[SparkyChick]

if you have something like a neon in circuit, your IR reading will be something like 0.02 Meg.Ohm, which, on that range is virtually Zero. however, a continuity reading would be more like 20K Ohms, which is a significant value.

It's worth pointing out that if it is a neon in circuit, on a continuity measurement it should not register at all as the voltage used for the test is no where near high enough to strike the neon so it will be non-conductive and appear as an infinite resistance... assuming it's working properly and hasn't failed with a dead short.

On the other hand if you have LED based indicators on accessories, they can cause readings on a continuity test because they will conduct at around 0.3v. If you think this is the case you can reverse the polarity of the test leads at which point, unless it's a dual chip LED, it should disappear (just in case... diodes will only conduct in one direction so reversing the polarity can take them out of the equation).

 

[newfutile]

Would an LED on presumably a high value resistor read anything at all, if it uses a capacitive dropper it wont read anything on a extra low voltage DC .

 

[SparkySy]

USB power sockets are another good one for misleading results due to the internal gubbins for the 5v outputs. They still catch me out from time to time!